1. Field of the Present Invention
The present invention relates to a tire air pressure estimating apparatus and, more particularly, to a tire air pressure estimating apparatus which estimates a friction state estimation value representing friction state between a tire and a road surface and which estimates a reduction of the tire air pressure based on the estimated friction state estimation value, without using a sensor for directly measuring the tire air pressure.
2. Description of the Related Art
For the purpose of detecting tire air pressure, it has been proposed to provide a pressure sensor in the tire to detect the tire air pressure directly. However, this method of directly detecting the tire air pressure has a problem in that a cost increase results from the need to provide a sensor in each tire. Further, when a vehicle is traveling at high speed, detection signals from the sensors can not be efficiently transmitted to the body of the vehicle, which makes it difficult to detect the tire air pressures.
Japanese Patent Application Laid-Open (JP-A) No. 8-164720 disclosed a technique of detecting an increase in angular velocity of rotation of a tire (wheel angular velocity), to thereby detect any reduction in the dynamic load radius of the tire, which is attributable to a reduction in the tire air pressure. This technique is now described in more detail. The following relationship exists between wheel angular velocity xcfx89, vehicle speed v, slip rate s (xe2x88x921 less than s less than 0 during driving), and dynamic load radius r during driving.
s=(vxe2x88x92rxcfx89)rxcfx89xe2x80x83xe2x80x83(1)
Equation (1) can be re-arranged as follows with respect to the wheel angular velocity.
xcfx89=v/{(1+s)r}xe2x80x83xe2x80x83(1xe2x80x2)
Therefore, in a low speed range at which great driving force is not required, the slip rate is small, so that a reduction of the dynamic load radius is detected as an increase in the wheel angular velocity. It is, for example, possible to detect a reduction in air pressure of one of left and right wheels caused by a puncture or the like, by detecting a difference between the angular velocities of the wheels.
However, as will be understood from the relationship shown in FIG. 1 between tire air pressure, frictional force between the tire and a road surface (which is equivalent to a driving force), and slip rate, the slip rate is changed significantly by a reduction in tire air pressure during high speed travel at which a greater driving force is required to overcome air resistance, compared to change of the slip rate during low speed travel at which this great driving force is not required. As the slip rate s (xe2x88x921 less than s less than 0) consequently approaches 0, the term (1+s) in Equation (1xe2x80x2) increases.
Accordingly, during high speed travel, the effect of a reduction in the dynamic load radius attributable to the reduction in tire air pressure and the effect of the increase of the term (1+s) are opposed by each other, and the reduction in the tire air pressure does not noticeably affect the wheel angular velocity of the driving wheel during high speed travel. This results in a problem that the accuracy of detection of the reduction in the tire air pressure of the driving wheel is reduced during high speed travel at which the greater driving force is required.
The present invention has been conceived to solve the above problem, and it is an object of the present invention to provide a tire air pressure estimating apparatus capable of estimating tire air pressure of a driving wheel, even during high speed travel, without using a sensor that detects the tire air pressure directly.
The principle of the present invention will now be described. An increase in ground contact area of a tire as a result of a reduction in tire air pressure appears as a change in friction state between the tire and a road surface. As shown in FIG. 2, which is similar to FIG. 1, the gradient of a tangent of a curve representing a relationship between a frictional force between the tire and the road surface and a slip rate (or slip speed), which is a xcexc-gradient of the road surface, increases as the tire air pressure decreases. That is, the xcexc-gradient of the road surface increases as the tire air pressure decreases.
An increase in the xcexc-gradient of the road surface can be estimated such that xcexc-gradient of the road surface is estimated using a road surface xcexc-gradient estimating technique, which is based on a wheel deceleration model, and an increase thereof is estimated. Alternatively, an increase in the xcexc-gradient of the road can be estimated by estimating an increase in break point frequency that results from the increase in the xcexc-gradient of the road surface, as will be described later, or by detecting a reduction in a vibration level at a special frequency. A gradient of braking force is represented by a gradient of a tangent of a curve that represents a relationship between slip speed (or, slip rate) and braking force. A gradient of driving force applied to the tire is represented by a gradient of a tangent of a curve that represents a relationship between slip speed (or, slip rate) and driving force. The gradient of braking force and the gradient of driving force are both physical quantities representing slipperiness between the tire and the road surface, or an estimation value of friction state representing friction state between the tire and the road surface. The gradient of braking force and the gradient of driving force are physical quantities equivalent to the xcexc-gradient of the road surface, which represents grip state of the tire. Therefore, estimation of an increase in the xcexc-gradient of the road surface will be described by describing estimation of an increase in the gradient of braking force.
As shown in FIG. 3, a dynamic model of a wheel resonance system can be represented by a model in which torsional spring elements 14 and 16 of a tire, having respective spring constants K1 and K2, are interposed between a rim 10 and a belt 12 and in which a suspension element, provided by connecting a spring element 18 having a spring constant K3 in parallel with a damper 20, is interposed between the rim 10 and a vehicle body. In this model, a disturbance from the road surface (road surface disturbance) is transmitted from the belt 12 through the spring elements 14 and 16 to the rim 10, to affect a wheel speed xcfx89, and is transmitted to the vehicle body through the suspension element.
A description is now given of characteristics of transmission from road surface disturbance to the wheel speed for the braking force gradient, using a fifth order full wheel model, in which a first order wheel decelerating motion, second order longitudinal direction suspension resonance, and second order tire rotation resonance are integrated.
FIG. 4 is a gain diagram showing frequency responses from road surface disturbance to the wheel speed for ranges from a limit braking range to a low slip range where there is some margin for tire characteristics (for ranges from a range at which the braking force gradient is 300 Ns/m to a range at which the braking force gradient is 10000 Ns/m). That is, the diagram shows the relationship between frequency and gain of amplitude of the wheel speed with respect to amplitude of the road surface disturbance.
The wheel speed frequency characteristics in FIG. 4 indicate that, when the braking force gradient is relatively small, such as near the limit of friction force between a tire and a road, the gain is great in a low frequency range and is small in a high frequency range. Namely, for the range where the braking force gradient is small, there is a big difference between the gain in the low frequency range and the gain in the high frequency range.
In contrast, the gain in the low frequency range for the range where the braking force gradient is relatively large, such as a stationary traveling region, is much smaller compared to those for the range where the braking force gradient is relatively small, in the wheel speed frequency characteristics. Further, in the high frequency range, the gain for the range where the braking force gradient is relatively large is not much smaller than the gain for the range where the braking force gradient is relatively small because of the influence of generation of rotational resonance of the tire (near 40 Hz) or the like. Therefore, for the range where the braking force gradient is relatively large, there is only a small difference between the gain in the low frequency range and the gain in the high frequency range. A difference between the vibration level of wheel speed signal in the low frequency range and the vibration level of wheel speed signal in the high frequency range changes similarly to the difference between the low frequency range gain and the high frequency range gain.
It is apparent from the above that a difference between the low frequency range gain and the high frequency range gain or between the wheel speed signal vibration levels in the low frequency range and the wheel speed signal vibration levels in the high frequency range decreases as the braking force gradient increases. Utilizing this characteristic, an increase in the braking force gradient (an increase in the xcexc-gradient of the road surface) can be estimated from the above-described differences. The change in the xcexc-gradient of the road surface which is attributable to the change in the braking force gradient is described above, an increase in the xcexc-gradient which is attributable to an increase in ground contact length due to a decrease in tire air pressure can be detected similarly. Accordingly, it is made possible to estimate a reduction in the tire air pressure. In the above description, xe2x80x9cdifference (for example, the difference between gains)xe2x80x9d is used, but xe2x80x9cratio (for example, the ratio of gains)xe2x80x9d may just as well be used.
Referring to the frequency band near 40 Hz in FIG. 4 at which rotational resonance (torsional resonance) of the tire occurs, the greater the braking force gradient, the sharper the peak waveform of rotational resonance of the tire. Further, as the braking force gradient becomes greater, the overall frequency characteristics of the peak waveform moves to higher frequency range. Similarly, if the wheel characteristics are approximated by a first-order lag model, it can be understood that a break point frequency becomes higher as the braking force gradient becomes greater, as shown in FIG. 8.
It is therefore possible to estimate an increase in the braking force gradient (an increase in the road surface xcexc-gradient) from a break point frequency of transmission characteristics from the road disturbance to the wheel speed, by approximating the characteristics of the wheel with a first-order lag model and estimating the break point frequency, which is a frequency at which the gain changes from a value in a predetermined range to a value out of the predetermined range. This makes it possible to estimate a reduction in the tire air pressure. Lag models of the second and third orders and the like have characteristics substantially similar to those of the first-order lag model. Therefore, it is possible to estimate an increase in the braking force gradient from the value of the break point frequency in the same manner as described above, by approximating wheel characteristics with the lower order lag model and estimating the break point frequency of the transmission characteristics of the wheel.
Also, in the PSD (Power Spectral Density) of wheel speed signal as shown in FIG. 25, torsional resonance in frequency near 40 Hz is small in high speed travelling, and resonance characteristics is scarcely observed when the tire air pressure is normal. On the other hand, when the tire air pressure decreases, the vibration level of the wheel speed signal, especially in a range from an unsprung resonance frequency to a torsional resonance frequency, decreases. Accordingly, referring to high frequency cut off characteristics of a low frequency range including frequency lower than the torsional resonance frequency, the gain in a high frequency range which range is higher than the low frequency range is relatively increase (When the tire air pressure decreases, because the gain in the low frequency range including frequency lower than the torsional resonance frequency decreases). Accordingly, the tire air pressure can be estimated by referring the change in the break point frequency and the change in the vibration level of wheel speed signal, namely, the change in frequency cut off characteristics.
Further, in a case of a plurality of wheels, because it is assumed that respective input disturbance levels from the road surface of the plurality of wheels are the same, an air pressure reduction of at least one of the plurality of wheels can be determined by comparing changes of the plurality of wheels (change in frequency cut off characteristics in frequency range including frequency lower than the torsional resonance frequency, which is attributable to an increase in ground contact length of the road surface and the tire due to generation of an tire air pressure reduction, in a wheel speed frequency characteristics).
It is preferable that the high frequency cut off characteristics to be referred is, from FIG. 25, a high frequency cut off characteristics in frequency range including frequency lower than the torsional resonance frequency. It is more preferable that the high frequency cut off characteristics to be referred is a high frequency cut off characteristics in frequency range which is lower than or equal to the torsional resonance frequency and more than or equal to an unprung resonance frequency. Instead of the high frequency cut off characteristics, a change in vibration levels is used. It is preferable that the vibration level to be referred is the vibration level in frequency range including frequency lower than the torsional resonance frequency, more preferably, in frequency range which is lower than or equal to the torsional resonance frequency, further preferably, in frequency range which is lower than or equal to the torsional resonance frequency and more than or equal to an unprung resonance frequency.
The present invention has been conceived based on the above-described principle. A configuration in a first aspect of the present invention includes a wheel speed sensor for detecting a wheel speed; a road surface friction state estimator for estimating a friction state estimation value which represents friction state between a road surface and a tire on the basis of the wheel speed detected by the wheel speed sensor; and a tire air pressure estimator for estimating pressure of the tire on the basis of the friction state estimation value estimated by the road surface friction state estimator. A configuration in a second aspect of the present invention includes a wheel speed sensor for detecting a wheel speed of a single wheel; a road surface friction state estimator for estimating a friction state estimation value, which represents friction between a tire and a road surface, based on the wheel speed detected by the wheel speed sensor; and a tire air pressure estimator for estimating pressure of the tire on the basis of the friction state estimation value estimated by the road surface friction state estimator.
A configuration in a third aspect of the present invention includes wheel speed sensors for detecting respective wheel speeds of a plurality of wheels; road surface friction state estimators for, on the basis of the respective wheel speeds of the plurality of wheels detected by the wheel speed sensors, estimating friction state estimation values, which represent friction states between a tire and a road surface; and a tire air pressure estimator for estimating pressures of the tires on the basis of the friction state estimation values estimated by the road surface friction state estimators. The plurality of wheels may be left and right front wheels or left and right rear wheels or a combination of one rear wheel and a front wheel in a diagonal relationship therewith.
When the wheel speed of the single wheel is detected, the road surface friction state estimator in each aspect of the present invention can estimate, as the friction state estimation value, one of: (a) in a transmission characteristics, from road surface disturbance to the wheel speed, approximated to a low order model, a frequency, at which gain changes from a value in a predetermined range to a value out of the predetermined range, in a gain diagram representing a frequency response of the approximated model; and (b) one of a difference or ratio between a characteristic quantity in a low frequency range including frequency lower than a torsional resonance frequency and a characteristic quantity in a high frequency range which is higher than the low frequency range, in a gain diagram representing a frequency response of a transmission characteristics, from the road surface disturbance to the wheel speed. As the characteristic quantity, vibration level of a wheel speed signal may be used. Also, the road surface friction state estimator can also estimate the level of vibration in the wheel speed signal in each frequency band as the friction state estimation value.
When the wheel speeds of the plurality of wheels are detected, the surface friction estimator can estimate, as the friction state estimation value for each of the plurality of the wheels, at least one of: (a) in a transmission characteristics, from road surface disturbance to the wheel speed, approximated to a low order model, a frequency, at which gain changes from a value in a predetermined range to a value out of the predetermined range, in a gain diagram representing a frequency response of the approximated model; (b) a vibration level of a wheel speed signal in a special frequency band; and (c) a vibration level of the wheel speed signal in a special frequency range, the wheel speed signal being obtained by processing using frequency-weighted filter which makes gain high at a higher frequency range of the special frequency range.
A first-order lag model may be used as the above-described lower order model.
As described above, friction state estimation values (such as one of a difference or ratio between a characteristic quantity in a low frequency range including frequency lower than a torsional resonance frequency and a characteristic quantity in a high frequency range which is higher than the low frequency range, in a gain diagram representing a frequency response of a transmission characteristics, from the road surface disturbance to the wheel speed, in a transmission characteristics, from road surface disturbance to the wheel speed, approximated to a low order model, a frequency (break point frequency), at which gain changes from a value in a predetermined range to a value out of the predetermined range, in a gain diagram representing a frequency response of the approximated model, a vibration level of a wheel speed signal in a special frequency band, braking force gradient, driving force gradient, and road surface xcexc-gradient) are related to the air pressure of a tire, and the friction state estimation values increase when the tire air pressure decreases. It is therefore possible to estimate a reduction in the tire air pressure by estimating an increase in friction state estimation value based on the friction state estimation values.
When the pressures of the tires are estimated based on a difference or ratio between friction state estimation values of a plurality of wheels, as in the third aspect of the present invention, since influence of the road surface is eliminated by obtaining the difference or ratio, the air pressure of one wheel due to puncture or the like can be accurately estimated.
In the second aspect, it is possible that the road surface friction state estimator estimates, as the friction state estimation value, a change in frequency cut off characteristics in a frequency range including frequency lower than a torsional resonance frequency, which is attributable to an increase in ground contact length, in a wheel speed frequency characteristics. In the third aspect, it is possible that the road surface friction state estimator estimates, as the friction state estimation value, one of: a change in frequency cut off characteristics in a frequency range including frequency lower than a torsional resonance frequency, which is attributable to an increase in ground contact length, and a change in vibration suppression characteristics in the frequency range including frequency lower than the torsional resonance frequency, which is attributable to the increase in the ground contact length.
When a tire air pressure is estimated from a detected value of a single wheel, a configuration may be employed which includes: a wheel speed sensor for detecting wheel speed of a single wheel; a tire air pressure estimator for estimating pressure of a tire, on the basis of the estimated wheel speed detected by the wheel speed sensor, from a change in frequency cut off characteristics in a frequency range including frequency lower than a torsional resonance frequency, in a wheel speed frequency characteristics. A configuration may also be employed which includes a wheel speed sensor for detecting wheel speed of a single wheel; a detection section for detecting, based on the wheel speed signal that is output by the wheel speed sensor, a characteristic quantity of the wheel speed signal in a low frequency range including frequency lower than the torsional resonance frequency and a characteristic quantity of the wheel speed signal in a high frequency range which is higher that the low frequency range; and tire air pressure estimator for estimating pressure of a tire by comparing the characteristic quantities detected by the detection section with each other. A configuration may further be employed which includes a wheel speed sensor for detecting wheel speed of a single wheel; a detection section for detecting, based on the wheel speed signal that is output by the wheel speed sensor, a characteristic quantity of the wheel speed signal in each of a plurality of frequency ranges; and tire air pressure estimator for estimating pressure of a tire by comparing the plurality of characteristic quantities detected by the detection section with each other.
When tire air pressures are estimated based on the detected values of a plurality of wheels, A tire air pressure estimating apparatus may comprises: wheel speed sensors for detecting respective wheel speeds of a plurality of wheels; a tire air pressure estimator for estimating pressures of tires, based on the respective wheel speeds of the plurality of wheels that are output (detected) by the wheel speed sensors, from one of: a change in frequency cut off characteristics in a frequency range including frequency lower than a torsional resonance frequency of wheel speed frequency characteristics, and a change in vibration suppression characteristics in the frequency range including frequency lower than the torsional resonance frequency. A configuration may yet further be employed which includes wheel speed sensors for detecting respective wheel speeds of a plurality of wheels; detection sections, for detecting, based on the respective wheel speeds of the plurality of wheels detected by the wheel speed sensors, one of a characteristic quantity of a wheel speed signal in each of a plurality of frequency bands and a characteristic quantity of the wheel speed signal in a special frequency range, the wheel speed signal in the special frequency range being obtained by processing the wheel speed signal using a frequency-weighted filter which makes gain high at the special high frequency range of the frequency range, for each wheel; and tire air pressure estimator for estimating pressures of tires by comparing the respective characteristic quantities of the wheel speed signals detected by the detection section with each other. The vibration level of the wheel speed signal may be used as the characteristic quantity of the wheel speed signal.